1. Technical Field
The present invention relates to an image forming apparatus such as a printer and a copying machine, employing a simultaneous transfer and fixing method for transferring a toner image on a toner image carrier onto a recording medium and fixing the toner image on the recording medium during the transfer.
2. Prior Art
Conventionally, an image forming method has been widely used in which an electrostatic latent image is formed on a photosensitive member, and after it is developed with dry toner, the toner image is electrostatically transferred onto a recording medium for fixing. In such image forming method, paper, which is a recording medium, cannot be brought into tight contact with the photosensitive member completely because of irregularity on the surface of the paper, and uneven gaps are caused, thus causing disordered transfer electric field or Coulomb repulsion between toner themselves. This leads to a problem that the image is blurred.
In order to solve this problem, proposed are an image forming method in which a plurality of toner images having different colors are superposedly transferred electrostatically on an intermediate transfer, member, and after those multi-colored multiple toner images are further melted on the intermediate transfer member, the multiple toner images thus melted are transferred and fixed on a recording medium to obtain a color image, and an image forming method in which a toner image on a photosensitive member formed into the shape of an endless belt is melted, and the toner image thus melted is transferred and fixed on a recording medium to obtain an image. According to the latter method, it is difficult to cause such deteriorated image quality as described above because the toner image is non-electrostatically transferred onto the recording medium.
In such an image forming method, in order to improve the transfer of a toner image onto a recording medium from an intermediate transfer member or a photosensitive member, it is proposed to cool and set the toner while the intermediate transfer member or the photosensitive member, the toner image and the recording medium are laid one on top of another, and thereafter to separate the intermediate transfer member or the photosensitive member from the toner and the recording medium. According to such a method, since the toner sets along the surface of the intermediate transfer member or the photosensitive member, it is possible to obtain a highly lustrous image having a feeling of high grade by smoothing the surface of the intermediate transfer member or the photosensitive member.
In such an image forming method, however, an excellent, uniformly lustrous image without disordered pixels can be obtained at low speed in which sufficient heating time is given, but it has been found that at high speed, in which sufficient heating time is not given, very little uneven luster at a pitch of about 0.3 mm to 2 mm occurs although there is no blurred image. In addition, such uneven luster was not recognized when the intermediate transfer member or the photosensitive member was separated from the toner and the recording medium immediately after the intermediate transfer member or the photosensitive member, the toner and the recording medium are laid one on top of another for heating and compressing as described above.
Such uneven luster is generally known as "microgross unevenness" in fixing. It is known that this phenomenon occurs because the surface of a fixing roll which comes into contact with toner has high hardness, uniform heating and compression are not performed on the toner, but it is unevenly melted. This is solved by making the hardness of the surface the surface of the fixing roll lower.
In the transfer fixing method in which after a multi-colored multiple toner image is melted on an intermediate transfer member or a photosensitive member, the multiple toner image thus melted is transferred onto a recording medium to obtain a color image, when the hardness of the elastic layer on the surface of the intermediate transfer member which comes into contact with the toner is reduced, it was recognized that the uneven luster described above is rather worsened. Problems to be solved by the Invention:
The present invention has been achieved in the light of the above-described problems, and its object is to provide an image forming apparatus, such as a printer and a copying machine, employing a simultaneous transfer and fixing method for transferring a toner image carried on a toner image carrier, such as an intermediate transfer member or a photosensitive member, for carrying a toner image, onto a recording medium and fixing the toner image on the recording medium during the transfer, capable of obtaining a high image quality of image having high glossiness without uneven luster and disordered pixels over the entire area from the highlight area to the medium density portion and the high density portion. PA1 an endless toner image carrier for carrying a toner image on the surface thereof while circulating, and for conveying the toner image to a predetermined transfer fixing unit; PA1 recording medium conveying means for conveying a recording medium to the transfer fixing unit in synchronism with timing at which the toner image on the toner image carrier is conveyed to the transfer fixing unit; and heating and compression means having, in the transfer fixing unit, a first roll, which comes into contact with the back of the toner image carrier, and a second roll for compressing the toner image carrier and the recording medium, which sandwich the toner image therebetween, between the first roll and the second roll, for heating and compressing the toner image by these first and second rolls, for conveying the recording medium to a predetermined separation position by the toner image carrier while the recording medium, which has passed through a location whereat it is sandwiched between the first and second rolls, is being maintained in a state being in contact with the toner image carrier, and separating the recording medium from the toner image carrier to thereby form an image fixed on the recording medium, PA1 the first roll and the second roll have a surface layer respectively, and the toner image carrier has a surface image on the side in contact with the recording medium, PA1 in a nip portion formed when the first roll and the second roll are pressed against each other, each diameter, and the material and thickness of each surface layer of the first roll and the second roll are selected so that a plane or a concave surface is formed on a part of the nip portion in the concave side roll of one of the first and second rolls, and PA1 assuming the outside diameter of the other convex side roll is different from the concave side roll of the first and second rolls to be R (mm), the hardness of a hardness test of durometer type A and the thickness of the surface layer of the concave side roll to be HR (degree) and dR (mm) respectively, the hardness of a hardness test of durometer type A and the thickness of the surface layer of the toner image carrier in contact with the recording medium to be Hb (degree) and db (.mu.m) respectively, and the nip width, which is the width of the toner image carrier in the movement direction, of the nip portion to be N (mm), the material and thickness of each surface layer of the first roll, the second roll and the toner image carrier are selected so that the following relation is satisfied: EQU (Hb/HR)&gt;0.8566 Ln{(N.sup.2 /R).multidot.(db/dR)}-0.5077